63 research outputs found
New approximate solutions of the Ginsparg-Wilson equation - tests in 2-d
A new method for finding approximate solutions of the Ginsparg-Wilson
equation is tested in 2-d. The Dirac operator is first constructed and then
used in a dynamical simulation of the 2-flavor Schwinger model. We find a very
small mass of the pi-particle implying almost chirally symmetric fermions. The
generalization of our method to 4-d is straightforward.Comment: Revised version (to appear in Physics Letters B); references added
and regrouped; new comment on finite size effect
2+1 flavor QCD with the fixed point action in the -regime
We generated configurations with the approximate fixed-point Dirac operator
on a lattice with fm where the scale
was set by . The distributions of the low lying eigenvalues in different
topological sectors were compared with those of the Random Matrix Theory which
leads to a prediction of the chiral condensate.Comment: 6 pages, 5 figures, presented at the XXV International Symposium on
Lattice Field Theory, July 30 - August 4 2007, Regensburg, German
Opening a nodal gap by fluctuating spin-density-wave in lightly doped LaSrCuO
We investigate whether the spin or charge degrees of freedom are responsible
for the nodal gap in underdoped cuprates by performing inelastic neutron
scattering and x-ray diffraction measurements on LaSrCuO, which
is on the edge of the antiferromagnetic phase. We found that fluctuating
incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass
dispersion exists even in this magnetic sample. The strongest component of
these fluctuations diminishes at the same temperature where the nodal gap
opens. X-ray scattering measurements on the same crystal show no signature of
charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the
electronic band of this cuprate opens due to fluctuating SDW with no
contribution from CDW
Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2
We report elastic neutron scattering and transport measurements on the Ni and
Cr equivalently doped iron pnictide BaFeNiCrAs.
Compared with the electron-doped BaFeNiAs, the long-range
antiferromagnetic (AF) order in BaFeNiCrAs is
gradually suppressed with vanishing ordered moment and N\'{e}el temperature
near without the appearance of superconductivity. A detailed analysis
on the transport properties of BaFeNiAs and
BaFeNiCrAs suggests that the non-Fermi-liquid
behavior associated with the linear resistivity as a function of temperature
may not correspond to the disappearance of the static AF order. From the
temperature dependence of the resistivity in overdoped compounds without static
AF order, we find that the transport properties are actually affected by Cr
impurity scattering, which may induce a metal-to-insulator crossover in highly
doped BaFeNiCrAs.Comment: 10 pages, 12 figure
Improving the Dirac Operator in Lattice QCD
Recently various new concepts for the construction of Dirac operators in
lattice Quantum Chromodynamics (QCD) have been introduced. These operators
satisfy the so-called Ginsparg-Wilson condition (GWC), thus obeying the
Atiyah-Singer index theorem and violating chiral symmetry only in a modest and
local form. Here we present studies in 4-d for SU(3) gauge configurations with
non-trivial topological content. We study the flow of eigenvalues and we
compare the numerical stability and efficiency of a recently suggested chirally
improved operator with that of others in this respect.Comment: Contrib. to Conf. on Comp. Physics, Sept. 2001 (Aachen); 4 pages, 4
figures, (LaTeX style files cpauth.cls, elsart.cls
Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain
A current challenge in the field of magnetoelectric multiferroics is to
identify systems that allow a controlled tuning of states displaying distinct
magnetoelectric responses. Here we show that the multiferroic ground state of
the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial
strain. Neutron diffraction reveals that in highly strained films the magnetic
order changes from the bulk-like incommensurate bc-cycloidal structure to
commensurate magnetic order. Concomitant with the modification of the magnetic
ground state, optical second-harmonic generation (SHG) and electric
measurements show an enormous increase of the ferroelectric polarization, and a
change in its direction from along the c- to the a-axis. Our results suggest
that the drastic change of multiferroic properties results from a switch of the
spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric
magnetostriction in epitaxially-strained TbMnO3. These findings experimentally
demonstrate that epitaxial strain can be used to control single-phase
spin-driven multiferroic states
Competing superconducting and magnetic order parameters and field-induced magnetism in electron-doped \mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}
We have studied the magnetic and superconducting properties of Ba(Fe0.95Co0.05)2As2 as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modeling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments
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